arm64: dts: meson-axg: s400: add dmic codec

There are 7 digital mics on the MIC daughter board attached
to the s400 board, so add the digital microphone codec to
its DTS

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>